Pyrotechnic cloud seeding compositions are generally old and well known. These compositions have been widely used for artificially modifying the weather by injection, in a finely dispersed form such as a smoke, into clouds for purposes of creating ice freezing nuclei. The ice freezing nuclei have the effect of creating rain, reducing hail and even for preventing rain by overseeding. Extensive descriptions of the cloud seeding process and compositions therefor appear in the prior art including issued patents and published literature.
As the art of seeding clouds for weather modification purposes has developed, one of the principal compositions developed for that purpose is a pyrotechnic composition comprised of silver iodate, which upon the burning of the composition produces silver iodide artificial ice nuclei, an inorganic fuel such as the light metals aluminium and magnesium, and a suitable binder such as epoxy resin. The pyrotechnic compositions have been varied by the inclusion of various fuels, binders, oxidizing agents, alkali iodates, and other materials. The effectiveness of these compositions is generally measured in terms of the nucleation efficiency, that is the number of ice nuclei per gram of composition at the nucleation temperature. This temperature is generally in the range of 0.degree. to -20.degree. C. Although the compositions are generally more effective at the lower temperatures, many showing a marked decrease in effectiveness as the temperature approaches 0.degree. C.
The basic principles involved in cloud seeding involve the formation, in a super-cooled cloud, of artificial ice freezing nuclei. The artificial nuclei promote the precipitation of moisture in the form of rain from the super-cooled cloud. The effectiveness of the artificial ice nuclei generating composition is determined by the number of artificial ice nuclei produced per gram of active ingredients, usually silver iodide.
A wide variety of pyrotechnic compositions have been disclosed in the art, and the following patents are illustrative:
______________________________________ Issue Pat. No. Date Inventor Title ______________________________________ 3,046,168 7-24-62 L. A. Burkhardt Chemically Produced Colored Smokes 3,140,107 7-7-64 M. M. Williams Pyrotechnic Composition 3,375,148 3-26-68 W. G. Finnegan Pyrotechnics Comprising Silver Iodate, Ammonium Nitrate, Nitrocellulose and Nitrate Esters 3,418,184 12-24-68 R. F. Vetter Smoke Producing Propellant 3,630,950 12-28-71 H. M. Papee Combustible Compositions (now Re. For Generating Aerosols, 29,142) Particularly Suitable For Cloud Modification and Weather Control and Aerosolization Process 3,677,840 7-18-72 G. C. Shaw Pyrotechnics Comprising Oxide of Silver For Weather Modification Use 3,698,968 10-17-72 D. M. Johnson Pyrotechnic Composition 3,769,107 10-30-73 D. M. Johnson Pyrotechnic Composition For Generating Lead Based Smoke 3,802,971 4-74 L. A. Burkhardt Pyrotechnic Formulations For Weather Modification Comprising a Mixture of Iodates ______________________________________